1. Field of the Invention
This invention relates generally to a planetary gearing of an automatic transmission for a motor vehicle, and, more particularly to controlling a friction brake arranged in parallel with an overrunning brake.
2. Description of the Prior Art
Automatic transmissions typically include a one-way brake (OWB), sometimes referred to as overrunning brake, as a reaction element in first gear to produce non-synchronous 1-2 upshifts and 2-1 downshifts between first and second gears. The reaction element for first gear has a high torque ratio to input torque and therefore requires a large, heavy, costly one-way brake.
An increase is the number of transmission speed ratios increases the torque ratio on the reaction one-way brake. The torque ratios on the first gear reaction member for transmission having a range of speed ratios is a s follows: 4 speed˜1.8, 5-speed˜2.1, 6-speed 2.4, 8-speed˜2.7. The increased torque ratio requires significantly larger, heavier, and more costly one way brakes.
The first gear reaction one-way brake is usually arranged in parallel with a hydraulically-actuated friction brake, which is engaged in reverse gear and to provide engine braking in manually selected first gear operation. Although this brake may have sufficient torque capacity to carry first gear torque, it must be sized for its required torque ratio in reverse gear for the number of speed ratios produced by the transmission: 4-speed˜3.32, 5-speed˜3.8, 6-speed˜4.6, 8-speed˜5.9.